This invention relates to apparatus for sensing the movement of a movable element and, more particularly, to apparatus responsive to the movement of a movable element for generating an electrical signal containing information indicative of the direction, speed and distance of movement of said movable element.
Sensing apparatus of the above-described type is most often used in a feedback control system, such as that used to control the movement of a linearly movable or rotary element. Exemplary linearly movable elements are the carriages employed in disk drives and serial printers, whereas an exemplary rotary element is the print wheel employed in so-called "daisy-wheel" serial printers.
Most serial printers of the print wheel type currently on the market are either of the open loop stepper motor driven variety or the closed-loop d-c servo motor variety. In the open-loop system, separate stepper motors are used to drive the linear movement of the print carriage and the rotary movement of the print wheel mounted to the carriage. As these motors operate open loop, no sensing apparatus of the type above-described is required or employed. Open loop stepper motor control systems have the disadvantage of requiring relatively powerful and expensive stepper motors to insure stability and accuracy.
In the closed-loop d-c servo motor control systems currently employed in the great majority of serial impact printers of the print wheel type, an optical or electrically inductive transducer device is coupled to each of the carriage and print wheel d-c servo motors. The tranducer devices currently employed either directly or indirectly through accompanying logic circuits generate a pair of phase-displaced sinusoidal signals, which provide an indication of the direction, speed and distance of movement of the movable element driven by the associated d-c servo motor, i.e., the carriage or the print wheel.
Examples of electrically inductive transducer devices currently used in closed-loop d-c servo control systems for print-wheel type serial impact printers are disclosed in U.S. Pat. Nos. 3,839,665, 3,954,163, 4,047,086, 4,059,789 and 4,091,911. Reference may also be had to the HyType series of serial impact printers manufactured by Diablo Systems, Inc. of Hayward. Calif. Examples of optical transducer devices for closed-loop d-c servo control systems may be had by reference to the Sprint series of serial impact printers manfuactured by Qume Corporation of San Jose, Calif. While closed-loop d-c servo motor control systems employing optical or electrically inductive transducer devices offer the advantages of higher speed and greater accuracy than open loop stepper motor systems, they are considerably more costly.
The use of closed-loop control systems for stepper motors is also well-known. However, to date such systems have proven relatively complex and costly.
It would be desirable, therefore, to provide a relatively simple and low cost motion sensing device for sensing the movement of a movable element, wherein such motion sensing device could be used in a closed-loop stepper motor control system for controlling the movement of such movable element.